14520 square feet left to mow.
Create an equation where it is (half-acre)=(3/5)(lawn area). 21780 is a half acre in square feet.
21780=(3/5)x
multiply both sides by (5/3) to find the total lawn measurement
x = 36300
subtract the half-acre from the total lawn to find the remaining area to mow.
36300-21780=14520